The present invention relates to a method for generating steam from a black liquor in a digester plant of a chemical pulp mill.
Conventionally, fiberline systems have a chip bin where steaming of wood chips or other cellulose material occurs, liquid is added to form a slurry, followed by pressurization of the slurry (this section is also referred to as the feed system), fed to a treatment vessel or vessels (could be an impregnation vessel, a pre-hydrolysis process or other vessels), followed by a digester (this section is also referred to as the cooking system). Currently, at least one black liquor stream (typically at a temperature of 110-150° C.) is withdrawn from the cooking system. The extracted black liquor stream or streams are used as a source of heat to “pre-heat” white liquor, other black liquor streams, and/or other liquid streams being sent to the feed and cooking systems. The extracted black liquor stream (or streams) is then sent to the pre-evaporation system, e.g., two or more flash tanks where steam is produced from the hot black liquor as the liquor is cooled, typically to temperatures of approximately 95-110° C. At this point, the black liquor is sent to the evaporator system in the recovery area. The flash steam so produced can be used elsewhere in the pulping process. For example, flash steam can be used directly to presteam chips prior to cooking.
The above flashing process, though it has been successfully employed in conventional continuous digesters, has the drawback that the steam produced contains volatile compounds, including sulfur compounds, which are undesirable in the presteaming of wood chips. Typically, wood chips are steamed at atmospheric pressure, or slightly above, such that the residual gases not absorbed by the wood chips must be collected and treated. The treatment typically is carried out by combustion in a mill's noncondensable gas (NCG) system. However, this collection and treatment system becomes particularly significant when the steam used contains volatile compounds, including sulfur compounds, which have undesirable environmental impact, including noxious odor. It is therefore preferable to use a source of steam which minimizes or eliminates the introduction of volatile compounds to the steaming process.
Steam is also needed for heating the fibrous material to the cooking temperature in the vapor phase of the digester. In the known systems medium-pressure steam from the mill's turbine plant is typically used for this purpose. Due to the cost of energy, any further improvement to the energy efficiency of the chemical pulp mill is needed.
U.S. Pat. No. 4,944,840 discloses a process in which waste liquor discharged from a digester is evaporated in multiple evaporation stages. Vapors generated in the evaporation stages are directly introduced to the impregnation and cooking zones in a digester for heating the fibrous material.
SE patent 453,673 reveals a method in which a fibrous material is cooked with a cooking liquor while passing steam into the top of the digester where the fibrous material and the liquor flow continuously and are separated, the cooking liquor being recycled. According to the method, a part of the cooking liquor is extracted from the digester and led to a steam converter for production of steam. The steam is fed to the digester in order to heat the fibrous material introduced to the desired temperature.
A system is revealed in U.S. Pat. No. 6,722,130 for the generation of pure steam from black liquor. The pressure of the black liquor is first reduced in order to produce a second black liquor at a higher concentration and black liquor vapor, which is condensed to form a condensate. The condensate is heated by the first black liquor and expansion evaporated to produce pure steam which is used in a chip bin.
A system is revealed in U.S. Pat. No. 6,176,971 for the generation of pure steam to be used in the chip bin. Substantially clean useable steam is produced from a hot spent treatment liquor (e.g. black liquor) by passing the spent liquor to a reboiler, and then pressurizing (e.g. with an eductor, fan, or compressor) the clean steam discharged from the reboiler. The quantity of clean steam produced is increased by placing under negative pressure the pure steam side of the steam converter in the steam converter with a steam-driven ejector. The reduced pressure of the pure steam side ensures that more heat can be withdrawn from the black liquor, which in itself gives a greater quantity of steam, while the supply of steam to the ejector also contributes to the delivery of greater quantities of steam. In this case, however, the steam vapor formed consists of a mixture of pure steam that has been expelled from the process fluid and steam that has been taken from the steam supply network of the mill for driving the ejector.
A further process for the generation of pure steam is revealed in U.S. Pat. No. 6,306,252 for use in the chip bin, where the black liquor from the digester is led through a heat exchanger in which pure process water is heated, after which the pressure of the heated process water is reduced, such that pure steam is generated.
US Patent Application Publication 2007/0131363 discloses a method which comprises generating black liquor in a digester system, sending the black liquor to an evaporator system without using any pre-evaporator system, flashing the black liquor in the evaporator system to yield steam, and using at least some of the steam for chip steaming in a chip bin and/or for supplying in-direct heat exchangers in the digester system for pre-heating white liquor and/or filtrates for use in the digester system.
WO 2007073333 discloses a system and a method for the generation of steam in a digester plant for the production of chemical cellulose pulp. The pressure of hot, pressurized black liquor from a digester is reduced in a first step for the formation of black liquor steam that is used for the steam pre-treatment of the chips in a second pre-heating step. Pure steam for the steam pre-treatment of the chips in a first preheating step is formed through re-heating the black liquor the pressure of which has been reduced before a final subsequent pressure reduction, where the increased quantity of black liquor steam is led to a steam converter for the generation of pure steam.
WO 2008/057040 concerns a method which comprises an impregnation vessel in which to impregnate the chips, which chips are then fed to a subsequent digester vessel in a transfer fluid. A black liquor withdrawal is taken from the digester, which withdrawal is led to the bottom in order there to heat the chips before they are fed out from the impregnation vessel. A withdrawal of the transfer fluid is taken from the top of the digester and led to a position in order there to act as impregnation fluid in the impregnation vessel. At least a portion of the transfer fluid that was withdrawn from the top of the digester passes through an indirect heat exchanger, in which the transfer fluid withdrawn from the top of the digester at a temperature exchanges heat indirectly with a first fluid for the production of steam from the first fluid. The steam that is produced is then led to a steam pre-treatment position, upstream of the impregnation process, in order to heat the chips at said steam pretreatment position.
The known solutions provide different systems for producing cleaner steam for heating needs in the digester plant and for improving the energy economy of the pulp mill.